1. Field of the Invention
The present invention relates to a height detecting apparatus for detecting the height of a workpiece such as a semiconductor wafer held on a chuck table included in a processing machine such as a laser processing machine.
2. Description of the Related Art
In a semiconductor device fabrication process, a plurality of crossing division lines are formed on the front side of a substantially disk-shaped semiconductor wafer to thereby define a plurality of separate regions where a plurality of devices such as ICs and LSIs are respectively formed. The semiconductor wafer is cut along the division lines to thereby divide the regions where the devices are formed from each other, thus obtaining individual semiconductor chips.
As a method of dividing the semiconductor wafer as a workpiece along the division lines, there has been tried a laser processing method using a pulsed laser beam having a transmission wavelength to the wafer, wherein the pulsed laser beam is applied to the wafer in the condition where the focal point of the pulsed laser beam is set inside the wafer in an area to be divided. A wafer dividing method using this laser processing method includes the steps of applying a pulsed laser beam having a transmission wavelength (e.g., 1064 nm) to the wafer in the condition where the focal point of the pulsed laser beam is set inside the wafer from one side thereof, thereby continuously forming a modified layer inside the wafer along each division line, and next applying an external force to the wafer along each division line where the strength has been reduced by the formation of the modified layer, thereby dividing the workpiece (see Japanese Patent No. 3408805, for example). In the case of forming a modified layer inside a workpiece along each division line by using a laser beam as mentioned above, it is important to set the focal point of the laser beam inside the workpiece at a predetermined depth from the upper surface of the workpiece.
As another method of dividing a platelike workpiece such as a semiconductor wafer, there has been proposed a method including the steps of applying a pulsed laser beam to the workpiece along the division lines formed on the workpiece, thereby forming a laser processed groove on the workpiece along each division line, and next breaking the workpiece along each laser processed groove by using a mechanical breaking apparatus (see Japanese Patent Laid-open No. 2010-272697, for example). Also in the case of forming such a laser processed groove along each division line formed on the workpiece as mentioned above, it is important to set the focal point of the laser beam at a predetermined height in the workpiece.
However, a platelike workpiece such as a semiconductor wafer has undulation, so that there are variations in thickness of the workpiece. Accordingly, it is difficult to perform uniform laser processing. More specifically, in the case of forming a modified layer inside a wafer along each division line, variations in thickness of the wafer may cause a problem such that the modified layer cannot be uniformly formed at a predetermined depth in relation to refractive index in applying a laser beam. Further, also in the case of forming a laser processed groove along each division line on a wafer, variations in thickness of the wafer may cause a problem such that the laser processed groove cannot be formed at a uniform depth. To solve these problems, Japanese Patent Laid-open No. 2011-82354 has disclosed a height detecting apparatus capable of detecting the height of the upper surface of a workpiece such as a semiconductor wafer held on a chuck table.